(a) Field of the Invention
The present invention relates to a catalyst system used for preparing polyolefin and a method for polymerizing olefin using the same.
(b) Description of the Related Art
A metallocene catalyst having cyclopentadienyl ligand as a basic structure is widely used for olefin polymerization. Such a metallocene catalyst requires anionic co-catalyst as a catalyst activator. And, the development of homogeneous catalyst having active, single-site exceeding the range of the conventional metallocene catalyst is actively progressed (G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem., Int. Ed. Eng. 38 (1999) 429). Most active, single-site catalysts as well as metallocene catalysts consist of ion pair of cation and anion. The part where polymerization practically occurs is cationic central metal and it has electrophilic character due to cationic property to increase metal-ethylene bond interaction. In order to show high polymerization activity, anion should not be coordinated with cationic central metal. As an anionic active cocatalyst, compounds having large volume such as methylaluminoxane (MAO) or tris(pentafluorophenyl)boron are used, and when anions derived from these compounds are used, they are not coordinated with central metal and thus high activity can be obtained.
However, in a catalyst in which cation and anion exist as an ion pair, anion turns round central metal to block approach of monomers to center metal, and thus it may inhibit polymerization.
Recently, in order to overcome this, attempts to prepare zwitterion catalyst having anion and cation in one molecule have been reported. The following Equations 1 and 2 show the representative examples.
In Equation 1 a compound in which one part of substituent of cyclopentadienyl is bound to zirconium through methylene group is reacted with tris(pentafluorophenyl)boron to obtain activated zwitterion catalyst (W. E. Piers, et al. J. Am Chem. Soc. 1997, 5132). The structure of the catalyst is similar to that of metallocene catalyst having cyclopentadienyl as a basic structure, except that anion exists in one molecule by covalent bond. 
In Equation 2, when carboxamidato-Nickel(II) that has a neutral ligand is reacted with B(C6F5)3, Lewis acid B(C6F5)3 is attached to an oxygen atom of carbonyl of the ligand to provide activated catalyst in the form of zwitterion compound (Lee, B. Y. et. al. J. Am. Chem. Soc. 2001, 123, 5352). 
However, conventional catalyst consisting of ion pair mainly uses MAO or tris(pentafluorophenyl)boron in activation step, which are expensive and thus increase cost.
Accordingly, it is an object of the present invention to provide zwitterion catalysts with various structures through novel reactions, and to provide a method for polymerizing olefin using the same.
Specifically, it is an object of the present invention to provide a catalyst having novel molecular structure and polymer having novel molecular structure.